Preheat oven to 180C or 160C (fan).

Line two large baking trays with baking parchment.

Add the chocolate and butter to a small heatproof bowl, place over a saucepan of simmering water. Make sure that the bowl does not touch the water. Heat until all the butter and chocolate has melted. Alternatively melt in the microwave.

In a large mixing bowl, add the flour, cocoa powder and baking powder. Mix together and set aside.

In a stand mixer, add the caster sugar, egg whites and espresso coffee. Beat together, until the egg whites are stiff and fluffy.

Add the melted chocolate and butter to the egg white mixture, and gently fold together.

Add the chocolate and egg white mixture to the mixing bowl with the flour and cocoa powder, and gently mix together. Be careful not to over beat the mixture.

Stir in the chocolate chunks.

Using a tablespoon, place dollops of the mixture onto the baking tray. Make sure the mixture dollops are spaced well apart to allow them to spread when they are baking.

Pop them in the oven for 10 minutes.

Remove from the oven to cool on a wire rack, then try not to eat them all at once.